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wikipedia - Pulse-width modulation From Wikipedia the free...

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Pulse-width modulation From Wikipedia, the free encyclopedia Jump to: navigation , search An example of PWM: the supply voltage (blue) modulated as a series of pulses results in a sine-like flux density waveform (red) in a magnetic circuit of electromagnetic actuator. The smoothness of the resultant waveform can be controlled by the width and number of modulated impulses (per given cycle) Pulse-width modulation (PWM) of a signal or power source involves the modulation of its duty cycle , to either convey information over a communications channel or control the amount of power sent to a load. Contents [ hide ] 1 Principle o 1.1 Delta o 1.2 Sigma-Delta o 1.3 Digital o 1.4 Types o 1.5 Spectrum 2 Applications o 2.1 Telecommunications o 2.2 Power delivery o 2.3 Voltage regulation o 2.4 Audio effects and amplification 3 See also 4 External links o 4.1 Applications [ edit ] Principle Fig. 1: a square wave , showing the definitions of y min , y max and D. Pulse-width modulation uses a square wave whose pulse width is modulated resulting in the variation of the average value of the waveform. If we consider a square waveform f ( t ) with a low value y min , a high value y max and a duty cycle D (see figure 1), the average value of the waveform is given by:
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As f ( t ) is a square wave, its value is y max for and y min for . The above expression then becomes: This latter expression can be fairly simplified in many cases where y min = 0 as . From this, it is obvious that the average value of the signal () is directly dependent on the duty cycle D. Fig. 2: A simple method to generate the PWM pulse train corresponding to a given signal is the intersective PWM: the signal (here the green sinewave) is compared with a sawtooth waveform (blue). When the latter is less than the former, the PWM signal (magenta) is in high state (1). Otherwise it is in the low state (0). The simplest way to generate a PWM signal is the intersective method, which requires only a sawtooth or a triangle waveform (easily generated using a simple oscillator ) and a comparator . When the value of the reference signal (the green sine wave in figure 2) is more than the modulation waveform (blue), the PWM signal (magenta) is in the high state, otherwise it is in the low state. [ edit ] Delta Main article: Delta modulation The output signal is compared with limits, which correspond to a reference signal offset by a constant. Every time the output signal reaches one of the limits, the PWM signal changes state.
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